Thermal and catalytic conversion of hydrocarbons to olefins is an important industrial process producing billions of pounds of olefins each year. Because of the large volume of production, small improvements in operating efficiency translate into significant profits. Catalysts play an important role in more selective conversion of hydrocarbons to olefins.
Particularly important catalysts are found among the natural and synthetic zeolites. Zeolites are complex crystalline aluminosilicates which form a network of AlO.sub.4 and SiO.sub.4 tetrahedra linked by shared oxygen atoms. The negative charge of the tetrahedra is balanced by the inclusion of protons or cations such as alkali or alkaline earth metal ions. The interstitial spaces or channels formed by the crystalline network enable zeolites to be used as molecular sieves in separation processes. The ability of zeolites to adsorb materials also enables them to be used in catalysis. There are a large number of both natural and synthetic zeolitic structures. The wide breadth of such structures may be understood by considering the work "Atlas of Zeolite Structure Types" by W. M. Meier, D. H. Olson and C. H. Baerlocher (4th edn., Butterworths/Intl. Zeolite Assoc. [1996]). Catalysts containing zeolite have been found to be active in cracking hydrocarbons to ethylene and propylene, the prime olefins. Of particular interest are the ZSM-5 zeolite described and claimed in U.S. Pat. No. 3,702,886, and ZSM-11 described in U.S. Pat. No. 3,709,979, and the numerous variations on these catalysts disclosed and claimed in later patents.
There is a constant need for increasing yields in conversion of hydrocarbons to ethylene and propylene, and especially for increasing the yields of propylene relative to ethylene in catalytic hydrocarbon processing. As global petroleum supplies are depleted, the need for improved yield will become increasingly important. The prior art has not filled the need for improved yield, although there have been many attempts. The present invention provides improved conversion of hydrocarbons to light olefins, and especially propylene by deliberately providing diolefins in a hydrocarbon feed subjected to catalytic conversion. As one can see, the prior art teaches away from the claimed invention, showing at best maintenance of ethylene yield.
Adams, U.S. Pat. No. 3,360,587, teaches separation of ethylene from acetylene, butadiene and other contaminants contained in the effluent from the thermal cracking of saturated hydrocarbons by introduction of the effluent into the reaction stream of a heavy oil catalytic cracking process, with the overall objective of increasing gasoline boiling components. Adams reports the recovery of the ethylene fraction with reduced acetylene and butadiene content, but shows a decrease in conversion to propylene. Also Adams did not use modern zeolite catalysts, especially those of the ZSM-5 or ZSM-11 types nor did Adams observe a significant increased yield of ethylene over separate thermal and catalytic cracking steps. Adams' reported yield comparison showed 80.9 mols (2263 lb.) of ethylene for the separate streams compared to 81.8 mols. (2295 lb.) of ethylene (32 lb., 1.3% net increase) from the stream having butadiene and acetylene combined with the heavy oil feed in the catalytic cracking operation. Adams viewed the result as conserving the ethylene, not an enhanced yield (See Adams col. 7 lines 24-26 ". . . obviously indicating that none of the ethylene from the pyrolysis effluent is `lost` in the catalytic cracking zone."). Adams did not observe that the addition of diolefins to a feed stream could substantially enhance conversion to light olefins including propylene.
Catalyst stability is an important factor in overall yield. In refinery operations crude oil is fractionated to produce feedstock streams for further treatments. The streams so produced are often referred to as "virgin" streams, when used without further processing. Because demand for the lower molecular weight hydrocarbons exceeds the demand for high molecular weight streams, many higher molecular weight fractions are cracked to lower molecular weight streams by thermal or catalytic cracking. These "cracked" streams share the boiling range and major components with "virgin" streams of the same designation as for example "light cat naphtha" (LCN) indicating a catalyst cracked naphtha as compared to "light virgin naphtha" (LVN). While these streams have similar boiling ranges and include some of the same components, they often have quite different performance in refinery operations. For example it has long been recognized that catalyst life in zeolite cracking is substantially greater when processing LVN streams than when processing cracked streams such as LCN. On the other hand LCN streams often exhibit higher initial conversions to ethylene and propylene. The present invention provides a method for enhancing LVN yields to levels similar to those obtained with LCN, while delaying the loss of catalyst stability observed with LCN.
In summary the art continues to seek improved yield of light olefins, but the process of the present invention has not previously been recognized.